The invention relates to a process for the hydrodesulfurization and deoxygenation of a gas simultaneously comprising oxygen and organic sulfur compounds.
Natural gas, as distributed by pipe network, contains traces of organic sulfur compounds resulting from an incomplete desulfurization or voluntarily introduced to give a smell to the gas (usually tetrahydrothiophene (THT) or mercaptans). Although present in the gas at a low concentration (for example 10 to 30 mg THT per Normal cubic meter of gas), these sulfur compounds are a nuisance when the gas is used in the presence of sulfur-sensitive catalysts. It is especially the case when the gas is used to manufacture hydrogen or synthesis gas by stream reforming, for example in ammonia or methanol producing plants.
The gas is usually desulfurized by hydrogen treatment in the presence of a hydrodesulfurization catalyst to convert the organic sulfur compounds to hydrogen sulfide which can be easily separated or retained by, for example, a zinc oxide mass. The catalysts to be used in the hydrodesulfurization step usually comprise cobalt and molybdenum or nickel and molybdenum on an alumina carrier.
The network gas sometimes also contains small amounts of air added to adjust the combustion power of too rich gases. The oxygen content is usually from 0.05 to 1% by volume. The hydrodesulfurization catalysts do not work properly in these conditions: a portion of sulfur is converted to elemental sulfur and/or sulfur dioxide which are not retained by the zinc oxide mass and poison the catalysts used in the subsequent treatment of the gas.
Other gases containing both oxygen and organic sulfur compounds (for example COS, CS.sub.2, mercaptans, sulfides, disulfides) are, for example, obtained by coking and gasification of coal, lignite, bituminous shales or oil residues, or consist of the associated gases of oil wells or refinery gases.
Two methods have been proposed to obviate this disadvantage.
According to U.S. Pat. No. 4,181,503, oxygen is removed in a first step by treatment of a mixture of the gas with hydrogen on a platinum-containing catalyst; then, in a second step, the oxygen-free gas is treated on a nickel-molybdenum catalyst to convert organic sulfur to hydrogen sulfide. This process has the disadvantage of being costly since, on the one hand, it requires a platinum catalyst and, on the other hand, the successive passage of the two catalysts cannot always be performed inside an already existing reactor only designed for the hydrodesulfurization of an oxygen-free gas.
The French patent application No. 82 02 550, now French Pat. No. 2,521,448, discloses the simultaneous performance of these two operations by treatment of the gas, after hydrogen addition (or more advantageously addition of an industrial gas comprising both hydrogen and carbon monoxide), on a palladium-containing catalyst.